Wireless mobile communication devices may be configured to maintain connectivity to a wireless network while they move, or are moved, in a coverage area of a wireless network. Connectivity may be maintained to cells comprised in the wireless network, each cell corresponding roughly to a certain cell coverage area.
As the wireless device moves, or roams, in the coverage area it may seek to remain connected to an optimal cell at each time. As the wireless device moves, the optimal cell can change as the wireless device moves from one cell coverage area to the next. In order to discover the optimal cell, the wireless device may conduct measurements of signal strengths transmitted by cell beacons or reference signals broadcasted by the cell, for example. When the wireless device discovers that a strength of a received signal associated with a cell the wireless device is attached to becomes weaker than that of a signal associated with another cell, the wireless device may be configured to report this to the wireless network to trigger handover to the cell with the stronger signal.
In some deployments, wireless networks comprise different types of cells. For example, large cells, known also as macro cells, may be used to serve highly mobile users and provide large-area coverage with a smaller number of cells. Similarly, small cells may be used to provide improved coverage or higher capacity to certain areas, more limited in size, where communication is frequently needed.
Femtocells, or in general small cells, may be deployed for various reasons. Smaller cells in general have been used to increase network capacity by allowing frequencies to be re-used to a higher degree over a given geographic area. A large cell allows for a set of frequencies to be used only once, which limits capacity. By splitting a large cell into smaller ones that may operate with a lower transmit power, the same set of frequencies may be used more than once, allowing a network operator to serve a larger number of subscribers in the same area and by using the same frequency band. Other names for small cells include picocells which may be seen as larger than femtocells, and microcells which in turn may be seen as larger than picocells.
Small cells may also be used where transmission power limits don't allow installing a cell of larger radius. This is due to the fact that a base station serving a large cell must transmit to the edge of the large cell, which is further away than an edge of a smaller cell.
Another reason for using small cells is to allow a limited subset of subscribers to access an alternative cell, which may be comprised in a larger network, the larger network being open to a larger set of subscribers. Such a cell may be known as a closed subscriber group, CSG, cell. To define a CSG cell, a corresponding set of subscribers that have access to the cell may be defined. A CSG cell may be considered to be a special type of femto-pico- or microcell, for example. Alternatively access to a small cell may be open and traffic may be directed to the small cell based on a type of the traffic, for example.
In heterogeneous network deployment, macro cells and small cells may be deployed on different frequencies, even where they form part of the same operator's network. Macro cells may form an area of continuous coverage on one or more frequencies, or carriers, and small cells may provide local enhancements to capacity and/or coverage of the network on one or more other frequencies at certain areas. Thus, there may be no interference between macro cells and small cells due to frequency separation. In this case interference coordination is not necessary between macro cells and small cells and mobiles don't always need to connect to a strongest available cell to avoid interference.